System and method of operating multi-engine system

ABSTRACT

There is described a method of operating a multi-engine system of an helicopter. The multi-engine system has a first turboshaft engine having a first shaft, a second turboshaft engine having a second shaft, a gearbox having a clutch system, and a range of rotation speeds defined as a placarded zone. The method generally has: rotating the first and second shafts at a flight rotation speed above the placarded zone when clutched to a load; decreasing a rotation speed of the first shaft from the flight rotation speed to a first idle rotation speed above the placarded zone; unclutching the first shaft from the load during the decreasing; and subsequently to the decreasing and the unclutching, simultaneously decreasing the rotation speeds of the first shaft and of the second shaft to a second idle rotation speed below the placarded zone, the simultaneously decreasing including clutching the first shaft to the load.

TECHNICAL FIELD

The present disclosure generally relates to multi-engine systems foraircraft and more particularly to methods and systems of operating suchsystems.

BACKGROUND OF THE ART

Helicopters are often provided with at least two turboshaft engines.Typically, the turboshaft engines are coupled to a main rotor via acommon reduction gearbox having a clutch system. The turboshaft enginesmay have undesirable dynamic vibrations modes that can be excited when ashaft rotates at speeds within a range of rotation speeds referred to asa placarded zone. In some embodiments, helicopters are provided withdedicated systems for decelerating a given one of the turboshaft engineswhen it's shaft is unclutched from the main rotor and free to rotate atrotation speeds within the placarded zone. For instance, in some ofthese dedicated systems, vanes within the given turboshaft engine areactuated from a low resistance position to a high resistance positionthereby decelerating the rotation speed of the shaft below the placardedzone, thereby avoiding unwanted vibrations. Although existing systemsfor decelerating turboshaft engines below the placarded zone aresatisfactory to a certain degree, there remains room for improvement.

SUMMARY

In one aspect, there is provided a method of operating a multi-enginesystem of an helicopter, the multi-engine system having a firstturboshaft engine having a first shaft, a second turboshaft enginehaving a second shaft, and a gearbox having a clutch system clutching atleast one of the first shaft and the second shaft to a load of thehelicopter, the first and second shafts having a range of rotationspeeds defined as a placarded zone, the method comprising: rotating thefirst and second shafts at a flight rotation speed above the placardedzone when the first and second shafts are clutched to the load;decreasing a rotation speed of the first shaft from the flight rotationspeed to a first idle rotation speed above the placarded zone;unclutching the first shaft from the load during the decreasing therotation speed of the first shaft; and subsequently to the decreasingand the unclutching, simultaneously decreasing the rotation speeds ofthe first shaft and of the second shaft to a second idle rotation speedbelow the placarded zone, the simultaneously decreasing includingclutching the first shaft to the load.

In another aspect, there is provided a system for operating amulti-engine system of an helicopter, the multi-engine system having afirst turboshaft engine having a first shaft, a second turboshaft enginehaving a second shaft, and a gearbox having a clutch system clutching atleast one of the first shaft and the second shaft to a load of thehelicopter, the first and second shafts having a range of rotationspeeds defined as a placarded zone, the system comprising: a processingunit; and a non-transitory storage medium having stored thereon programcode executable by the processing unit for: rotating the first andsecond shafts at a flight rotation speed above the placarded zone whenthe first and second shafts are clutched to the load; decreasing arotation speed of the first shaft from the flight rotation speed to afirst idle rotation speed above the placarded zone; unclutching thefirst shaft from the load during the decreasing the rotation speed ofthe first shaft; and subsequently to the decreasing and the unclutching,simultaneously decreasing the rotation speeds of the first shaft and ofthe second shaft to a second idle rotation speed below the placardedzone, the simultaneously decreasing including clutching the first shaftto the load.

In a further aspect, there is provided a method of operating amulti-engine system, the multi-engine system having a first enginehaving a first shaft, a second engine having a second shaft, and agearbox having a clutch system clutching at least one of the first shaftand the second shaft to a load, the first and second shafts having arange of rotation speeds defined as a placarded zone, the methodcomprising: rotating the first and second shafts at a flight rotationspeed above the placarded zone when the first and second shafts areclutched to the load; decreasing a rotation speed of the first shaftfrom the flight rotation speed to a first idle rotation speed above theplacarded zone; unclutching the first shaft from the load during thedecreasing the rotation speed of the first shaft; and subsequently tothe decreasing and the unclutching, simultaneously decreasing therotation speeds of the first shaft and of the second shaft to a secondidle rotation speed below the placarded zone, the simultaneouslydecreasing including clutching the first shaft to the load.

In a further aspect, there is provided a method of operating amulti-engine system of an helicopter, the multi-engine system having afirst turboshaft engine having a first shaft, a second turboshaft enginehaving a second shaft, and a gearbox having a clutch system clutching atleast one of the first shaft and the second shaft to a load of thehelicopter, the first and second shafts having a range of rotationspeeds defined as a placarded zone, the method comprising: rotating thefirst and second shafts at a first idle rotation speed below theplacarded zone when the first and second shafts are clutched to theload; increasing a rotation speed of the first shaft from the first idlerotation speed to a flight rotation speed, the flight rotation speedabove the placarded zone; unclutching the second shaft from the loadduring the increasing the rotation speed of the first shaft; andincreasing a rotation speed of the second shaft to a second idlerotation speed when the second shaft is unclutched from the load, thesecond idle rotation speed above the placarded zone and below the flightrotation speed.

In a further aspect, there is provided a system for operating amulti-engine system of an helicopter, the multi-engine system having afirst turboshaft engine having a first shaft, a second turboshaft enginehaving a second shaft, and a gearbox having a clutch system clutching atleast one of the first shaft and the second shaft to a common load ofthe helicopter, the first and second shafts having a range of rotationspeeds defined as a placarded zone, the system comprising: a processingunit; and a non-transitory storage medium having stored thereon programcode executable by the processing unit for: rotating the first andsecond shafts at a first idle rotation speed below the placarded zonewhen the first and second shafts are clutched to the load; increasing arotation speed of the first shaft from the first idle rotation speed toa flight rotation speed above the placarded zone; unclutching the secondshaft from the load during the increasing the rotation speed of thefirst shaft; and increasing a rotation speed of the second shaft to asecond idle rotation speed when the second shaft is unclutched from theload, the second idle rotation speed above the placarded zone and belowthe flight rotation speed.

In a further aspect, there is provided a method of operating amulti-engine system, the multi-engine system having a first enginehaving a first shaft, a second engine having a second shaft, and agearbox having a clutch system clutching at least one of the first shaftand the second shaft to a load, the first and second shafts having arange of rotation speeds defined as a placarded zone, the methodcomprising: rotating the first and second shafts at a first idlerotation speed below the placarded zone when the first and second shaftsare clutched to the load; increasing a rotation speed of the first shaftfrom the first idle rotation speed to a flight rotation speed above theplacarded zone; unclutching the second shaft from the load during saidincreasing the rotation speed of the first shaft; and increasing arotation speed of the second shaft to a second idle rotation speed whenthe second shaft is unclutched from the load, the second idle rotationspeed above the placarded zone and below the flight rotation speed.

In a further aspect, there is provided a method of operating amulti-engine system of an helicopter, the multi-engine system having afirst turboshaft engine having a first shaft, a second turboshaft enginehaving a second shaft, and a gearbox having a clutch system clutching atleast one of the first shaft and the second shaft to a load of thehelicopter, the first and second shafts having a range of rotationspeeds defined as a placarded zone, the method comprising:simultaneously i) rotating the first shaft at a flight rotation speedwhen the first shaft is clutched to the load and ii) rotating the secondshaft at a first idle rotation speed when the second shaft is unclutchedto the load, the first idle rotation speed below the flight rotationspeed and above the placarded zone; decreasing a rotation speed of thefirst shaft from the flight rotation speed to a given rotation speedwithin the placarded zone; decreasing a rotation speed of the secondshaft from the first idle rotation speed to the given rotation speedwithin the placarded zone; clutching the second shaft to the load whenthe rotation speed of the second shaft reaches the given rotation speedwithin the placarded zone; and simultaneously decreasing the rotationspeed of the first shaft and of the rotation speed of the second shaftto a second idle rotation speed below the placarded zone when the firstshaft and the second shaft are clutched to the load.

In a further aspect, there is provided a system for operating amulti-engine system of an helicopter, the multi-engine system having afirst turboshaft engine having a first shaft, a second turboshaft enginehaving a second shaft, and a gearbox having a clutch system clutching atleast one of the first shaft and the second shaft to a load of thehelicopter, the first and second shafts having a range of rotationspeeds defined as a placarded zone, the system comprising: a processingunit; and a non-transitory storage medium having stored thereon programcode executable by the processing unit for: simultaneously i) rotatingthe first shaft at a flight rotation speed when the first shaft isclutched to the load and ii) rotating the second shaft at a first idlerotation speed when the second shaft is unclutched to the load, thefirst idle rotation speed below the flight rotation speed and above theplacarded zone; decreasing a rotation speed of the first shaft from theflight rotation speed to a given rotation speed within the placardedzone; decreasing a rotation speed of the second shaft from the firstidle rotation speed to the given rotation speed within the placardedzone; clutching the second shaft to the load when the rotation speed ofthe second shaft reaches the given rotation speed within the placardedzone; and simultaneously decreasing the rotation speed of the firstshaft and of the rotation speed of the second shaft to a second idlerotation speed below the placarded zone when the first shaft and thesecond shaft are clutched to the load.

In a further aspect, there is provided a method of operating amulti-engine system, the multi-engine system having a first enginehaving a first shaft, a second engine having a second shaft, and agearbox having a clutch system clutching at least one of the first shaftand the second shaft to a load, the first and second shafts having arange of rotation speeds defined as a placarded zone, the methodcomprising: simultaneously i) rotating the first shaft at a flightrotation speed when the first shaft is clutched to the load and ii)rotating the second shaft at a first idle rotation speed when the secondshaft is unclutched to the load, the first idle rotation speed below theflight rotation speed and above the placarded zone; decreasing arotation speed of the first shaft from the flight rotation speed to agiven rotation speed within the placarded zone; decreasing a rotationspeed of the second shaft from the first idle rotation speed to thegiven rotation speed within the placarded zone; clutching the secondshaft to the load when the rotation speed of the second shaft reachesthe given rotation speed within the placarded zone; and simultaneouslydecreasing the rotation speed of the first shaft and of the rotationspeed of the second shaft to a second idle rotation speed below theplacarded zone when the first shaft and the second shaft are clutched tothe load.

DESCRIPTION OF THE DRAWINGS

Reference is now made to the accompanying figures in which:

FIG. 1 is a cross-sectional view of an example turboshaft engine of theaircraft in accordance with an illustrative embodiment;

FIG. 2A is a schematic view of a multi-engine aircraft, in accordancewith one or more embodiments;

FIG. 2B is a schematic diagram of an exemplary multi-engine system forthe aircraft of FIG. 2A, showing axial cross-sectional views of two gasturbine engines, in accordance with an illustrative embodiment;

FIG. 3 is a block diagram of an example architecture for operating amulti-engine aircraft, in accordance with an illustrative embodiment;

FIG. 4 is a flow chart of a method of operating a multi-engine aircraftin a first run-up phase, in accordance with one or more embodiments;

FIG. 5 is a graph showing commanded engine regime, fuel supply rate androtation speed as a function of time during the run-up phase of FIG. 4 ,in accordance with one or more embodiments;

FIG. 5A is a graph showing rotation speed of a second shaft as afunction of rotation speed of a first shaft, in accordance with anillustrative embodiment;

FIG. 6 is a flow chart of a method of operating a multi-engine aircraftin a first run-down phase, in accordance with one or more embodiments;

FIG. 7 is a graph showing commanded engine regime, fuel supply rate androtation speed as a function of time during the first run-down phase ofFIG. 6 , in accordance with one or more embodiments;

FIG. 8 is a flow chart of a method of operating a multi-engine aircraftin a second run-down phase, in accordance with one or more embodiments;

FIG. 9 is a graph showing commanded engine regime, fuel supply rate androtation speed as a function of time during the second run-down phase ofFIG. 8 , in accordance with one or more embodiments;

FIG. 10 is a schematic view of a controller of FIG. 3 , in accordancewith one or more embodiments;

FIG. 11A is a graph showing idle rotation speed of the second engine asa function of the measured rotation speed of the first engine, inaccordance with one or more embodiments; and

FIG. 11B is a graph showing idle rotation speed of the first engine as afunction of the measured rotation speed of the second engine, inaccordance with one or more embodiments.

DETAILED DESCRIPTION

There are described herein methods and systems for operating amulti-engine aircraft, which may be a multi-engine helicopter or otherrotorcraft. Under certain conditions, for instance when the multi-engineaircraft is operated from an idle regime to a flight regime, or viceversa, it can be desirable to operate the aircraft in a mannerpreventing any of its engines' shaft to dwell within a range of rotationspeeds defined as the placarded zone, which may excite undesirablevibration modes (e.g., bending modes, rigid modes, bounce modes) withinthe multi-engine aircraft. These vibration modes can be dynamicvibration modes due to supercritical shaft(s) of the multi-engineaircraft.

FIG. 2A depicts an exemplary multi-engine aircraft 100, which in thiscase is a helicopter. The aircraft 100 includes at least first andsecond gas turbine engines 102 and 104. The first and second gas turbineengines 102 and 104 can be interconnected, in the case of the depictedhelicopter application, by a common gearbox having a clutch system toform a multi-engine system 105 which drives a common load 107.

Turning now to FIG. 2B, illustrated is an example multi-engine system105 that may be used as a power plant for an aircraft, including but notlimited to a rotorcraft such as the helicopter 100. The multi-enginesystem 105 may include may not be limited to the first and second gasturbine engines 102 and 104 as the multi-engine system 105 can have morethan two gas turbine engines. In the case of a helicopter application,such as the one illustrated in FIG. 1B, the first and second gas turbineengines 102 and 104 are turboshaft engines (hereinafter “the engines 102and 104”).

More particularly, the first and second engines 102 and 104 areinterconnected by a common output gearbox 150 (also referred to hereinas a “main gearbox”) to drive a common load 107. In one embodiment, thecommon load 107 may comprise a rotary wing of a rotary-wing aircraft.For example, the common load 107 may be a main rotor 108 and a tailrotor 110 of the aircraft 100. Depending on the type of the common load107 and on the operating speed thereof, each of the engines 102 and 104may be drivingly coupled to the common load 107 via the output gearbox150, which may be of the speed-reduction type.

For example, the gearbox 150 may have a plurality of transmission shafts156 to receive mechanical energy from respective first and second shafts154 of respective first and second 102 and 104. The gearbox 150 may beconfigured to direct at least some of the combined mechanical energyfrom the first and second engines 102 and 104 toward a common outputshaft 158 for driving the common load 107 at a suitable operating (e.g.,rotation) speed. It is understood that the multi-engine system 105 mayalso be configured, for example, to drive accessories and/or otherelements of an associated aircraft. As will be described, the gearbox150 may be configured to permit the common load 107 to be driven byeither of the first and second engines 102 and 104 or, by a combinationthereof.

In particular, in one embodiment, the first and second shafts 154 of thefirst and second engines 102 and 104 may be connected through a coupling(not shown) to an input module (not shown) from the gearbox 150. Eachinput module may comprise a first reduction stage (not shown) and aclutch system 152, allowing each of the first and second engines 102 and104 to independently and individually clutch/declutch itself from thegearbox 150, and as such from any downstream load as in common load 107.For instance, the first shaft 154 of the first engine 102 may clutch tothe common load 107 when its rotation speed meet a rotation speed of thesecond shaft 154 of the second engine 102, or vice versa. The firstshaft 154 of the first engine 102 may declutch from the common load 107when its rotation speed differs from a rotation speed of the secondshaft 154 of the second engine 102. Both input modules may be connectedwithin the gearbox 150 and meet at a second reduction stage 155, thefirst reduction stage and the second reduction stage 155 having theirown reduction ratio. The common output shaft 158 may couple the secondreduction stage 155 to the tail rotor 110 while another shaft (notshown) may couple the second reduction stage 155 to the common load 107.

Control of the multi-engine system 105 is effected by one or morecontroller(s) 210 (hereinafter “the controller 210”), which may be FullAuthority Digital Engine Controller(s) (FADEC(s)), Electronic EngineController(s) (EEC(s)), Engine Control Unit(s) (ECU(s)), or the like,that are programmed to manage operation of the multi-engine system 105in the idle regime, in a flight regime and in a transient phases betweenthe idle regime and the flight regime. The first and second engines 102and 104 may be controlled by the controller 210 to move from an idleregime in which the first engine 102 runs at lower power conditions to aflight regime in which the first engine 102 runs at full (or near-full)power conditions, and vice versa. In some embodiments, the clutch system152 may be provided to permit each of the first and second engines 102and 104 to be clutched and de-clutched from the aircraft's main gearbox,as desired, depending on the regime in which the first and secondengines 102 and 104 are operated.

The controller 210 is configured to manage operation of the multi-enginesystem 105 in all transient phases of the typical aircraft mission. Thetransient phases of the typical aircraft mission can include, but arenot limited to, a full idle phase in which both engines 102 and 104 areoperated in an idle regime; a first run-up phase in which, while thefirst engine 102 remains in the idle regime, the second engine 104 ismoved to a flight regime; a second run-up phase in which, while thesecond engine 104 remains in the flight regime, the first engine 102 ismoved to the flight regime; a first run-down phase in which, while thefirst engine 102 remains in the flight regime, the second engine 104 ismoved to the idle regime; and a second run-down phase in which, whilethe second engine 104 remains in the idle regime, the first engine 102is moved to the idle regime as well. These transient phases can beencountered when the aircraft is on the ground, prior to take off and/orafter a landing. Although in some applications, some or all of thetransient phases can be encountered when the aircraft is in the air, offthe ground. As described in greater detail below, the controller 210 isconfigured to prevent the first and second shafts 154 of the first andsecond engines 102 and 104 from rotating at rotation speeds within theplacarded zone in the transient phases of the typical aircraft mission.It is noted that the terms first and second can be used interchangeably.For instance, the expressions “the first engine 102” and “the secondengine 104” can be used interchangeably with the expressions “one of theengines 102 and 104” and “the other one of the first and second engines102 and 104,” respectively.

The controller 210 may control the engines' governing on speed accordingto appropriate schedule(s) or control regime. The controller 210 maycomprise a first controller for controlling the first engine 102 and asecond controller for controlling the second engine 104. The firstcontroller and the second controller may be in communication with eachother in order to implement the operations described herein. In someembodiments, a single controller 210 may be used for controlling thefirst engine 102 and the second engine 104. Although variousdifferential control between the first and second engines 102 and 104 ofthe multi-engine system 105 are possible, in one particular embodimentthe controller 210 may correspondingly control fuel flow (i.e., massflow rate of fuel) to each of the first and second engines 102 and 104accordingly. Although the examples described herein illustrate twoengines, the system described herein is applicable to more than twoengines, whereby at least one of the multiple engines is clutched to thecommon load 107 while the remaining engines are declutched from thecommon load 107.

With reference to FIG. 1 , the first and second engines 102 and 104 canbe embodied as gas turbine engines. Although the foregoing discussionrelates to the first engine 102, it should be understood that the secondengine 104 can be substantively similar to the first engine 102. In thisexample, the engine 102 is a turboshaft engine generally comprising inserial flow communication a low pressure (LP) compressor section 12 anda high pressure (HP) compressor section 14 for pressurizing air, acombustor section 16 in which the compressed air is mixed with fuel andignited for generating an annular stream of hot combustion gases, a highpressure turbine section 18 for extracting energy from the combustiongases and driving the high pressure compressor section 14, and a lowerpressure turbine section 20 for further extracting energy from thecombustion gases and driving at least the low pressure compressorsection 12.

The low pressure compressor section 12 may independently rotate from thehigh pressure compressor section 14. The low pressure compressor section12 may include one or more compression stages and the high pressurecompressor section 14 may include one or more compression stages. Acompressor stage may include a compressor rotor, or a combination of thecompressor rotor and a compressor stator assembly. In a multistagecompressor configuration, the compressor stator assemblies may directthe air from one compressor rotor to the next.

The engine 102 has multiple, i.e., two or more, spools which may performthe compression to pressurize the air received through an air inlet 22,and which extract energy from the combustion gases before they exit viaan exhaust outlet 24. In the illustrated embodiment, the engine 102includes a low pressure spool 26 and a high pressure spool 28 mountedfor rotation about an engine axis 30. The low pressure and high pressurespools 26 and 28 are independently rotatable relative to each otherabout the axis 30. The term “spool” is herein intended to broadly referto drivingly connected turbine and compressor rotors.

The low pressure spool 26 includes a low pressure shaft 32interconnecting the low pressure turbine section 20 with the lowpressure compressor section 12 to drive rotors of the low pressurecompressor section 12. In other words, the low pressure compressorsection 12 may include at least one low pressure compressor rotordirectly drivingly engaged to the low pressure shaft 32 and the lowpressure turbine section 20 may include at least one low pressureturbine rotor directly drivingly engaged to the low pressure shaft 32 soas to rotate the low pressure compressor section 12 at a same speed asthe low pressure turbine section 20. The high pressure spool 28 includesa high pressure shaft 34 interconnecting the high pressure turbinesection 18 with the high pressure compressor section 14 to drive rotorsof the high pressure compressor section 14. In other words, the highpressure compressor section 14 may include at least one high pressurecompressor rotor directly drivingly engaged to the high pressure shaft34 and the high pressure turbine section 18 may include at least onehigh pressure turbine rotor directly drivingly engaged to the highpressure shaft 34 so as to rotate the high pressure compressor section14 at a same speed as the high pressure turbine section 18. In someembodiments, the high pressure shaft 34 may be hollow and the lowpressure shaft 32 extends therethrough. The two shafts 32 and 34 arefree to rotate independently from one another. The engine 102 mayinclude a transmission 38 driven by the low pressure shaft 32 anddriving a rotatable output shaft 40. The transmission 38 may vary aratio between rotational speeds of the low pressure shaft 32 and theoutput shaft 40.

As described hereinabove, control of the operation of the engine 102 canbe effected by one or more control systems, for example the controller210. The controller 210 can modulate a fuel flow provided to the engine102, the position and/or orientation of variable geometry mechanismswithin the engine 102, a bleed level of the engine 102, and the like. Insome embodiments, the controller 210 is configured for controllingoperation of multiple engines, for instance the first and second engines102 and 104. For example, the controller 210 can be provided with one ormore FADECs or similar devices. Each FADEC can be assigned to controlthe operation of one or more of the first and second engines 102 and104. Additionally, in some embodiments the controller 210 can beconfigured for controlling operation of other elements of the aircraft100, for instance the common load 107.

With reference to FIG. 3 , the aircraft 100, comprising the first andsecond engines 102 and 104 and the common load 107, is illustrated usinga block diagram. More than the two engines 102 and 104 may be present ona same aircraft 100 in some other embodiments. The first and secondengines 102 and 104 are mechanically coupled to the common load 107, forinstance as illustrated in FIG. 1B, for causing the common load 107 torotate and produce thrust for the aircraft 100. Although FIG. 3illustrates a singular common load 107, it should be noted that theaircraft 100 can include any number of rotors, including multiple mainrotors, one or more tail rotors, and the like. Collectively, the firstand second engines 102 and 104, and the common load 107 form part of themulti-engine system 105, which is controlled by the controller 210coupleable to one another via the gearbox and clutch system describedabove.

The multi-engine system 105 can be controlled by way of the controller210, as described hereinabove. The controller 210 can be composed ofvarious devices, including one or more FADEC(s), EEC(s), ECU(s), rotorcontroller(s), or any other suitable devices for controlling operationof the first and second engines 102 and 104, and/or the common load 107.In some embodiments, the operation of the first and second engines 102and 104, and of the common load 107 is controlled by way of one or moreactuators, mechanical linkages, hydraulic systems, and the like. Thecontroller 210 can be coupled to the actuators, mechanical linkages,hydraulic systems, and the like, in any suitable fashion for effectingcontrol of the first and second engines 102 and 104 and/or of the commonload 107. For example, if a change in the operating conditions of theaircraft 100 is detected without any corresponding change in inputs froman operator of the aircraft 100, the controller 210 can adjust inputsprovided to the first and second engines 102 and 104 and/or the commonload 107 in order to compensate for the uncommanded change.

One or more sensors 204 and 206 are coupled to the first and secondengines 102 and 104 for acquiring data about the operating parameters ofthe first and second engines 102 and 104. For instance, the sensors 204and 206 can monitor a respective rotation speed of the shafts of thefirst and second engines 102 and 104, and report the measured rotationspeeds to the system 202. Additionally, sensor(s) 208 are coupled to thecommon load 107 for acquiring data about the operating parameters of thecommon load 107. The sensor(s) 204, 206 and 208 may be any suitable typeof sensor used to measure (continuously or periodically) operatingparameters. The sensor(s) 204, 206 and 208 includes, but are not limitedto, speed sensors, acceleration sensors, pressure sensors, temperaturesensors, altitude sensors, weight on wheels sensors and the like. Thesensor(s) 204, 206 and 208, can be coupled to the controller 210 in anysuitable fashion, including any suitable wired and/or wireless couplingtechniques. In some embodiments, operating parameters may be receivedfrom one or more other sources, including but not limited to a FADEC, anECU, an EEC, or any related accessories that control any aspect ofengine performance. In some embodiments, measurements obtained from thesensor(s) 204, 206 and 208 are used to calculate or determine otherrelated parameters.

In some embodiments, the controller 210 is communicatively coupled toone or more power management switches (PMS) individually or collectivelycommanding operating regimes of the first and second engines 102 and104. For instance, in the illustrated embodiment, first and second PMS160 and 162 are commanding operation of the first and second engines 102and 104, respectively, through the controller 210. In the depictedembodiment, the first and second PMS 160 and 162 each have a physicalactuator such as a lever, a button, a knob and the like movable betweenan off position, an idle position and a fly position. In someembodiments, the first and second PMS 160 and 162 are provided in theform of graphical user interface(s) on a touch screen, for instance,where one or more digital buttons can allow actuation of the first andsecond PMS 160 and 162 as desired. In the off position, thecorresponding engine is off; in the idle position, the correspondingengine is ignited and commanded to operate in an idle regime; and in thefly position, the corresponding engine is commanded to operate in aflight regime. The first and second PMS 160 and 162 may be actuatedindependently by a pilot of the multi-engine aircraft 100. Accordingly,to command the full idle phase, the PMS 160 and 162 are both moved tothe idle position; to command the first run-up phase, one of the two PMS160 and 162 is moved from the idle position to the fly position; tocommand the second run-up phase, the other one of the PMS 160 and 162 ismoved from the idle position to the fly position; to command the firstrun-down phase, one of the PMS 160 and 162 is moved from the flyposition to the idle position; to command the second run-down phase, theother one of the PMS 160 and 162 is moved from the fly position to theidle position, and to command a shut down phase, the PMS 160 and 162 aremoved in the off position. The PMS 160 and 162 can be actuated by aflight crew member, or by the controller 210 depending on theembodiment.

Control of the multi-engine system 105 is effected by the controller210, as described herein below, which manages the operation of the firstand second engines 102 and 104 to prevent any one of the first andsecond engines 102 and 104 to dwell in the placarded zone during any ofthe transient phases of the typical aircraft mission. The risk ofdwelling in the placarded zone can happen in situations when one of thefirst and second engines 102 and 104 is unclutched from the common load107 and thereby free to rotate from an initial rotation speed to animpermissible rotation speed within the placarded zone. As describedbelow, such situations may arise in transient phases of the typicalaircraft mission such as the first run-up phase, the first run-downphase and the second run-down phase, for instance. An example method ofoperating the multi-engine system 105 to avoid dwelling in the placardedzone during the first run-up phase is described with reference to FIGS.4 to 5A. An example method of operating the multi-engine system 105 toavoid dwelling in the placarded zone during the first run-down phase isdescribed with reference to FIGS. 6 and 7 . An example method ofoperating the multi-engine system 105 to avoid dwelling in the placardedzone during the second run-down phase is described with reference toFIGS. 8 and 9 .

In twin engine arrangements, when the first and second engines 102 and104 are in the idle regime, the first and second shafts 154 are clutchedto the common load 107 and rotates at an idle rotation speed below theplacarded zone (hereinafter “the low idle rotation speed”). It isgenerally uncommon to simultaneously command both engines from the idleregime to the flight regime. Therefore, while the first engine 102remains in the idle regime, the second engine 104 is commanded into theflight regime first, as per the first run-up phase, which leads thefirst shaft 154 to unclutch and idle until its turn comes. However, itwas found that the first shaft 154 of the first engine 102, whenunclutched, is free to rotate more rapidly and may wander undesirablyinto the placarded zone, as shown by dashed line of bottom most graph ofFIG. 5 . To avoid this situation, it was found to provide a bit morefuel to a combustor section of the first engine 102 to bring therotation speed of the first shaft 154 from the low idle rotation speed,below the placarded zone, to a second idle rotation speed above theplacarded zone (hereinafter “the high idle rotation speed”) during thistransient process.

FIG. 4 shows a flow chart of a method 400 of operating the multi-enginesystem during the first run-up phase in which only one of the engines102 and 104 is commanded to the flight regime. The method 400 isdescribed with reference to FIG. 3 showing the multi-engine system 105.The method 400 is also described with reference to FIG. 5 showing agraph of the PMS positions associated with the first and second PMS 160and 162, fuel flow supplied to each one of the first and secondcombustor sections of the first and second engines 102 and 104, androtation speeds of the first and second shafts 154 during a full idlephase and a first run-up phase. It is noted that in the bottom-mostgraph of FIG. 5 , the solid lines show the nominal, programmed, desiredor otherwise reference rotation speeds for the first and second engines102 and 104 during the first run-up phase whereas the dotted lines showthe measured rotation speeds for the first and second engines 102 and104.

At step 402, the first and second shafts 154 are rotated at the low idlerotation speed below the placarded zone when the first and second shaftsare clutched to the common load 107. The other steps of the method 400can be initiated at any time when the first and second engines 102 and104 are in the full idle phase, i.e., when the first and second shafts154 are clutched to the common load 107 and rotating at the low idlerotation speed below the placarded zone. In this phase, for instance atmoment in time t0, the first and second PMS 160 and 162 are both at theidle position. As shown in this embodiment, the fuel flow that issupplied to each of the first and second combustor sections of the firstand second engines 102 and 104 are similar to one another, and therebythe first and second shafts 154 both rotate at a similar rotation speed,in this case the low idle rotation speed below the placarded zone. Insome embodiments, the method 400 is initiated upon receiving a commandto operate the multi-engine system 105 in a first run-up phase includingmoving the first engine in a flight regime while maintaining the secondengine in an idle regime. The method 400 can be initiated by a flightcrew member or by the controller 210, depending on the embodiment,moving the first PMS 160 from the idle position to the fly position.

At step 404, the rotation speed of the first shaft 154 is increased fromthe low idle rotation speed to the flight rotation speed, with theflight rotation speed being above the placarded zone. In this specificembodiment, a fuel flow to a first combustor section of the first engine102 is increased. The step 404 can be initiated at moment in time t1when the first PMS 160 associated with the first engine 102 is movedfrom the idle position to the fly position. As shown, the rotationspeeds of the first and second shafts 154 increase simultaneously asthey are both clutched to the common load 107. However, in this specificembodiment, as the fuel flow to the first combustor section of the firstengine 102 is greater than a fuel flow to the second combustor sectionof the second engine 104, the second shaft 154 may unclutch from thecommon load 107 when a difference in torque applied to the common load107 exceeds a given threshold.

At step 406, the second shaft 154 unclutches from the common load 107.The unclutching of the second shaft 154 occurs when a rotation speed ofthe first shaft 154 increases from the low idle rotation speed to aflight rotation speed above the placarded zone based on the step 404 ofincreasing the rotation speed of the first shaft of the first engine102. It is from that moment, e.g., moment in time t2, that a risk liesin letting the unclutched second shaft 154 freely rotate within theplacarded zone which may generate unwanted vibrations.

Instead of decelerating the rotation speed of the second shaft 154 usingexisting engine decelerating systems for decelerating the second shaft154 below the placarded zone, the method 400 has a step 408 in which therotation speed of the second shaft is increased to the high idlerotation speed above the placarded zone and below the flight rotationspeed. In some embodiments, the step 408 includes a step of increasing afuel flow to a second combustor section of the second engine 104. Forinstance, the step 408 can be performed upon the unclutching of thesecond shaft 154 or shortly thereafter. The step 408 can be initiated atmoment in time t1, at moment in time t2 or at any time between themoment in time t1 and the moment in time t2, depending on theembodiment. Maintaining the rotation speed of the unclutched shaft 154at the high idle rotation speed after moment in time t3 can prevent thesecond shaft 154 from rotating at impermissible rotation speeds withinthe placarded zone and thereby prevent undesirable vibration modes ofthe multi-engine system 105 to be excited. In some embodiments, it wasfound convenient to maintain the increased fuel flow, which may includea fuel flow FR_(F) to the first combustor section in order to maintainthe first engine's rotation at the flight rotation speed and a fuel flowFR_(HI) to the second combustor section to maintain the second engine'srotation at the high idle rotation speed, for the entirety of the firstrun-up phase of the aircraft mission. In this way, the first engine 102can be operated in the flight regime while the second engine 104 can beoperated in an idle regime and more specifically a high idle regimeabove the placarded zone.

The placarded zone is a zone in which undesirable vibration levelsoccur, or are considered likely to occur. The placarded zone can bepredefined in terms of a range of rotation speeds in data available toan engine controller and which the engine controller can compare tovalues or rotation speeds sensed by a rotation speed sensor, forinstance. In such a scenario, the engine controller can be configured tomove the current rotation speed just above the predefined placarded zonevalues upon detecting that the current value of rotation speed is withinthe placarded zone or is heading towards the placarded zone, forinstance. In an alternate embodiment, the placarded zone can bedefinable in real time by an engine controller based on vibrations, inwhich case the engine controller can have instructions to increase therotation speed when one or more vibration sensors provide an indicationof vibrations exceeding a given vibration threshold, for instance, andpotentially taking additional factors into consideration. Various otherimplementation modes are possible. In many various implementations, thelow idle rotation speed corresponding to the clutched state and belowthe placarded zone will be significantly lower, e.g. more than 10%lower, than the high idle rotation speed corresponding to the unclutchedstate and above the placarded zone. The flight rotation speed, alsocorresponding to a clutched state, will be significantly higher, e.g.more than 10% higher, than the high idle rotation speed.

Still referring to FIG. 5 , the range of rotation speeds of theplacarded zone can have a lower limit and an upper limit. In someembodiments, the lower and upper limits of the placarded zone of one ofthe first and second engines 102 and 104, typically the engine in theidle regime (“the idling engine”), are expressed relatively to arotation speed of the shaft of the other one of the first and secondengines, typically the engine in the flight regime (“the non-idlingengine”), or vice versa. In some embodiments, the placarded zone isabove a low idle rotation speed when the shaft is in a clutched state,below the flight rotation speed and below a high idle rotation speedwhen the shaft is in an unclutched state. The placarded zone includesrotation speeds where an unclutched shaft would dwell if its rotationspeed was not increased to the high idle rotation speed or decreased tothe low idle rotation speed using the methods and systems describedherein. For instance, in some embodiments, the low idle rotation speedis at 50% of the flight rotation speed. In these embodiments, theplacarded zone can have a range of about 10-30% of the flight rotationspeed and that range may extends between the low idle rotation speed andthe flight rotation speed, or between the low idle rotation speed andthe high idle rotation speed. Typically, the high idle rotation speedcan typically be set as close as possible to the upper limit of theplacarded zone to minimize fuel consumption while maintaining propersafety margins. The value ranges correspond to standard safety marginsfor at least some turboshaft engines, and can differ from one embodimentto another.

In some embodiments, the step 404 of increasing the rotation speed ofthe first shaft is initiated prior to initiation of the step 408 ofincreasing the rotation speed of the second shaft. In some otherembodiments, the step 404 of increasing the rotation speed of the firstshaft is initiated simultaneously to the step 408 of increasing therotation speed of the second shaft. In these latter embodiments, therate of change of the rotation speed of the first shaft can be greaterthan a rate of change of the rotation speed of the second shaft. In someembodiments, the rate of change in fuel flow to the first combustorsection can be quicker than a rate of change in fuel flow to the secondcombustion section. In some embodiments, the step 404 of increasing therotation speed of the first shaft is performed progressively and inaccordance with a first linear increase rate r1. The step 408 ofincreasing the rotation speed of the second shaft can be performedprogressively and in accordance with a second linear increase rate r2.In some embodiments, the first linear increase rate r1 can be steeperthan the second linear increase rate r2. For instance, the first linearincrease rate r1 can be at least twice the second linear increase rater2, depending on the embodiment. In these embodiments, a differencebetween the first linear increase rate and the second linear increaserate causes the first shaft 154 of the first engine 102 to acceleratemore quickly than the second shaft 154 which can lead to the unclutchingof the second shaft 154 of the second engine 104 that occurs at step406. In some embodiments, the step 408 of increasing the rotation speedof the second shaft is performed upon detecting that the second shafthas unclutched from the common load 107.

It is noted that the reference rotation speeds for the first and secondshafts 154 of the first and second engines 102 and 104, such as shown inthe bottom-most graph of FIG. 5 , can be programmed at the controller210. In these embodiments, the controller 210 can monitor, from any oneof the sensors 204 and 206 for instance, the rotation speeds of thefirst and second shafts 154 of the first and second engines 102 and 104and modify the fuel flow to the first and second combustor sections toensure that the measured rotation speeds correspond to the desiredrotation speeds plus or minus a given tolerance. In some embodiments,the controller 210 can monitor a pressure fluctuation signal or avibration pick-up level threshold to determine when to increase the fuelflow in order to exit from the placarded zone and stay above. In someembodiments, the controller 210 can increase or decrease the rotationspeed of a shaft within the engine rather than directly supplying morefuel flow. In some embodiments, minimum speed limit of other shafts andchange in variable geometry (e.g., variable guide vane, bleed valve). Insome embodiments, electric motor and/or generator can modulate powersupplied to the engine to control the rotation speed of the shaft, forinstance. In these latter embodiments, the power can be supplied eitheran electric starter or the electric motor of an hybrid configuration,for instance.

In a second run-up phase, the second engine may be brought to the flightregime by increasing the fuel flow to the second combustor section ofthe second engine 104. This increase may be initiated at a moment intime subsequent to moment in time t3 when the second PMS 162 associatedwith the second engine 104 is moved from the idle position to the flyposition. An example of the second run-up phase is shown in FIG. 5 aswell. Referring now to FIG. 5A, there is shown a graph of the rotationspeed of the unclutched second shaft as a function of the rotation speedof the clutched first shaft. As shown, as the rotation speed of theclutched first shaft increases from the low idle rotation speed to theflight rotation speed, the rotation speed of the unclutched second shaftincreases from the low idle rotation speed to the high idle rotationspeed.

In twin engine arrangements, it is generally desirable to bring thefirst and second engines 102 and 104 asynchronously from the fly regimeto the idle regime. However, the first one of the first and secondengines 102 and 104 to be moved from the flight regime to the idleregime may idle in the placarded zone. To avoid this, more fuel thanwould otherwise be required is supplied to the idling engine to keep itabove the placarded zone.

FIG. 6 shows a flow chart of a method 600 of operating the multi-enginesystem during the first run-down phase in which only one of the engines102 and 104 is commanded to the idle regime. The method 600 is describedwith reference to FIG. 3 showing the multi-engine system 105. The method600 is also described with reference to FIG. 7 showing a graph of thePMS positions associated with the first and second PMS 160 and 162, fuelflow supplied to each of the first and second combustor sections of thefirst and second engines 102 and 104, and rotation speeds of the firstand second shafts 154 during a full flight phase and a first run-downphase.

At step 602, the first and second shafts 154 are rotated at the flightrotation speed above the placarded zone when the first and second shafts154 are clutched to the common load 107. The other steps of the method600 can be initiated at any time when the first and second engines 102and 104 are in such a flight regime. In this phase, for instance atmoment in time t4, the first PMS 160 and 162 are both at the flightposition. As shown in this embodiment, the fuel flow that is supplied toeach of the first and second combustor sections of the first and secondengines 102 and 104 are similar to one another, and may correspond tofuel flow FR_(F) as introduced above, and thereby the first and secondshafts 154 both rotate at a similar rotation speed, in this case aflight rotation speed above the high idle rotation speed and above theplacarded zone. In some embodiments, the method 600 is initiated uponreceiving a command to operate the multi-engine system 105 in a firstrun-down phase including moving the first engine in an idle regime whilemaintaining the second engine in the flight regime. The method 600 canbe initiated by a flight crew member or by the controller 210, dependingon the embodiment.

At step 604, the rotation speed of the first shaft is decreased from theflight rotation speed to the high idle rotation speed above theplacarded zone. This step can be performed by decreasing a fuel flow tothe first combustor section of the first engine in some embodiments. Asshown, the step 604 can be initiated at moment in time t5 when the firstPMS 160 associated with the first engine 102 is moved from the flyposition to the idle position. In this transient phase, the second PMS162 is kept in the fly position. In some embodiments, the step 604 ofdecreasing the rotation speed of the first shaft 154 is simultaneous tomaintaining a rotation speed of the second shaft steady.

At step 606, the first shaft 154 unclutches from the common load 107during the decreasing of the rotation speed of the first shaft. In theillustrated embodiment, the unclutching of the first shaft 154 occurs atmoment in time t6, for instance, and it is from that moment in time thata risk exists in letting the first shaft 154 of the first engine freelyrotate which may cause it to wander in the placarded zone. It is notedthat, during the decreasing, a difference between a rotation speed ofthe first shaft and a rotation speed of the second shaft may cause theunclutching of the first shaft from the common load 107.

Subsequently to the steps 604 and 606, the method 600 has a step 608 ofsimultaneously decreasing the rotation speeds of the first and secondshafts 154 to the low idle rotation speed below the placarded zone. Thestep 608 includes a step of clutching the first shaft 154 to the commonload 107, as will be described further below.

When the rotation speed reaches the high idle rotation speed, forinstance at moment in time t7, the method 600 can maintain the fuelflow, such as fuel flow FR_(HI), to the first combustor section.Maintaining the fuel flow to the first combustor section, such as fuelflow FR_(HI), greater than would otherwise be required for the low idlerotation speed, such as the low idle fuel flow NL_(LI) shown in FIG. 7 ,can ensure that the rotation speed of the first shaft 154 remains at thehigh idle rotation speed above the placarded zone and thereby avoidunwanted vibrations in the first engine 102 during the first run-downphase.

The range of rotation speeds of the placarded zone can have a lowerlimit and an upper limit such as those described above with reference toFIG. 5 . As the idle rotation speed of one of the first and secondshafts can change depending on the actual rotation speed of the otherone of the first and second shafts, the method 600 can include a step ofmonitoring the rotation speeds of the first and second shafts in realtime. In these embodiments, the fuel flow supplied to each of the firstand second combustor sections of the first and second engines 102 and104 may depend on the measured rotation speeds of the first and/orsecond shafts as monitored. Accordingly, the method 600 can include astep of comparing the measured rotation speeds to reference rotationspeeds and a step of adjusting the fuel flow to the first combustorsection or other engine parameters based on the step of comparing.

In some embodiments, the step 604 of decreasing the rotation speed ofthe first shaft is performed progressively. This progressive decreasemay not be constant. For instance, the progressive decrease can includea first decrease rate r3 and a second decrease r4 subsequent to anddifferent from the first decrease rate r3, such as shown in FIG. 7 . Inthis embodiment, once the first shaft 154 unclutches from the commonload 107, a lesser fuel flow is required as torque to the common load107 is no longer applied. Accordingly, a lesser fuel flow is requiredfor the first shaft to continue its decrease in rotation speed. In theillustrated embodiment, the second decrease rate r4 is steeper than thefirst decrease rate r3. The second decrease rate r4 may be twice thefirst decrease rate r3 in some embodiments. The second decrease rate r4can be applied upon detecting that the first shaft 154 has unclutchedfrom the common load 107. It is noted that, in some embodiments, thefuel flow supplied to the first combustor section of the first engineduring step 604 may wander lower than the fuel flow required to maintainthe rotation speed of the first shaft to the high idle rotation speed.In these embodiments, a step of maintaining the fuel flow to the firstcombustor section can be provided, and may include a step of increasingthe fuel flow to the first combustor section back to the fuel flowFR_(HI) required to maintain the rotation speed of the first shaft tothe high idle rotation speed. An example of such a fuel flow curve isshown as a dotted line in the middle graph of FIG. 7 .

From a situation where the first engine 102 is in the idle regimerotating at the high idle rotation speed and the second engine 104 is inthe flight regime rotating at the flight rotation speed, it may bedesirable to bring the first and second engines 102 and 104 to the idleregime together. One may not simply move the second engine 104 from theflight regime to the idle regime because this may lead the second engine104 to unclutch and potentially idle in the placarded zone. Tocircumvent such as undesirable situation, it was found convenient todecrease the fuel flow to both the first and second engines 102 and 104simultaneously, but not at the same rate. In this way, the fuel flowdecreases to the second engine 104, the formerly “fly” or non-idlingengine, is steeper than the fuel flow decrease rate to the first engine102, the formerly “high idle” or idling engine, to allow the firstengine 102 to re-clutch while maintaining the clutching of the secondengine 104.

FIG. 8 shows a flow chart of a method 800 of operating the multi-enginesystem during the second run-down phase in which the first and secondengines 102 and 104 are commanded to the idle regime. The method 800 isdescribed with reference to FIG. 3 showing the multi-engine system 105.The method 800 is also described with reference to FIG. 9 showing agraph of the PMS positions associated with the first and second PMS 160and 162, fuel flow supplied to each of the first and second combustorsections of the first and second engines 102 and 104, and rotationspeeds of the first and second shafts 154 during a second run-downphase. It is noted that in the bottom-most graph of FIG. 9 , the solidlines show the nominal, programmed, desired or otherwise referencerotation speeds for the first and second engines 102 and 104 during thefirst run-down phase whereas the dotted lines show the measured rotationspeeds for the first and second engines 102 and 104.

At step 802, the first shaft is rotated at the flight rotation speedwhen the first shaft is clutched to the common load 107 and the secondshaft is rotated at the high idle rotation speed when the second shaftis unclutched to the common load 107. The other steps of the method 800can be initiated at any time when the first engine 102 is in the flightregime and the second engine 104 is in the idle regime. In this phase,for instance at moment in time t8, the first PMS 160 is in the flyposition and the second PMS 162 is in the idle position. As shown inthis embodiment, the fuel flow FR_(HI) that supplied to the firstcombustor section of the first engine 102 is greater than the fuel flowFR_(F) supplied to the second combustor section of the second engine104, and thereby the first and second shafts 154 rotate at the flightrotation speed and the high idle rotation speed, respectively. In someembodiments, the method 800 is initiated upon receiving a command tooperate the multi-engine system 105 in a second run-down phase includingmoving the first engine 102 in the idle regime as well. The method 800can be initiated by a flight crew member or by the controller 210,depending on the embodiment.

At step 804, the rotation speed of the first shaft is decreased from theflight rotation speed to a given rotation speed within the placardedzone. At step 806, the rotation speed of the second shaft is decreasedfrom the high idle rotation speed to the given rotation speed within theplacarded zone. In some embodiments, the decrease in rotation speed ofthe second shaft can be more quickly than the decrease in rotation speedof the first shaft. As shown, the steps 804 and 806 can be initiated atmoment in time t9 when the first PMS 160 associated with the firstengine 102 is moved from the fly position to the idle position. In someembodiments, the decrease in rotation speed can be performed by acorresponding decrease in fuel flow to the corresponding combustionsections of the first and second engines. For instance, the decrease infuel flow to the first combustor section can be simultaneous to thedecrease in fuel flow to the second combustor section.

At step 808, such as schematically illustrated at moment in time t10,the second shaft clutches to the common load 107 when a rotation speedof the second shaft 154 reaches the given rotation speed within theplacarded zone.

At step 810, the rotation speed of the first shaft and the rotationspeed of the second shaft are simultaneously decreased to the low idlerotation speed below the placarded zone when the first and second shaftsare clutched to the common load 107. In some embodiments, the step 810is performed upon detecting that the second shaft has clutched to thecommon load 107.

In some embodiments, the decrease of the fuel flow to the firstcombustor section is performed progressively and in accordance with afirst linear decrease rate r5. The decrease of the fuel flow to thesecond combustor section is performed progressively and in accordancewith a second linear increase rate r6. In some embodiments, the firstlinear increase rate r5 is twice as steep as the second linear increaserate r6. This difference between the first linear increase rate r5 andthe second linear increase rate r6 can cause the clutching of the secondshaft to the common load 107, as it forces the rotation speeds of thefirst and second shafts 154 to meet at the given rotation speed withinthe placarded zone. Although the rotation speeds of the first and secondshafts 154 meet within the placarded zone, the first and second shafts154 are both clutched to the main load 107 and can accordingly be forcedbelow the placarded zone by decreasing their fuel flow valuessimultaneously.

It is noted that the methods described above can be implemented by asystem comprising a processing unit, and a non-transitory storage mediumhaving stored thereon program code executable by the processing unit forperforming the steps of the methods described above. In someembodiments, the system is provided in the form of the controller 210.In other embodiments, the system is provided in the form of a controllerexternal to the controller 210 but in communicative coupled therewith.Accordingly, the system can directly or indirectly control the fuel flowsupplied to the first and second engines, receive sensor data from anyone of the sensors of the multi-engine system 105 and inputs from thefirst and second PMS switches for instance. It is noted that the methodsdescribed above may be performed only when the controller 210 determinesthat the aircraft is on ground. For instance, the controller 210 may becommunicatively coupled to one or more sensor(s) detecting whetherweight is imparted on the wheels on the aircraft.

The controller 210 can be provided as a combination of hardware andsoftware components. The hardware components can be implemented in theform of a computing device 1000, an example of which is described withreference to FIG. 10 . Moreover, the software components of thecontroller 210 can be implemented in the form of one or more softwareapplications (not shown).

Referring to FIG. 10 , the computing device 1000 can have a processor1002, a memory 1004, and I/O interface 1006. Instructions 1008 forperforming the methods 400, 600 and 800 described above with referenceto FIGS. 4, 6 and 8 can be stored on the memory 1004 and accessible bythe processor 1002.

The processor 1002 can be, for example, a general-purpose microprocessoror microcontroller, a digital signal processing (DSP) processor, anintegrated circuit, a field programmable gate array (FPGA), areconfigurable processor, a programmable read-only memory (PROM), or anycombination thereof.

The memory 1004 can include a suitable combination of any type ofcomputer-readable memory that is located either internally or externallysuch as, for example, random-access memory (RAM), read-only memory(ROM), compact disc read-only memory (CDROM), electro-optical memory,magneto-optical memory, erasable programmable read-only memory (EPROM),and electrically-erasable programmable read-only memory (EEPROM),Ferroelectric RAM (FRAM) or the like.

Each I/O interface 1006 enables the computing device 1000 tointerconnect with one or more input devices, such as TBD, or with one ormore output devices such as TBD.

Each I/O interface 1006 enables the controller TBD to communicate withother components, to exchange data with other components, to access andconnect to network resources, to serve applications, and perform othercomputing applications by connecting to a network (or multiple networks)capable of carrying data including the Internet, Ethernet, plain oldtelephone service (POTS) line, public switch telephone network (PSTN),integrated services digital network (ISDN), digital subscriber line(DSL), coaxial cable, fiber optics, satellite, mobile, wireless (e.g.Wi-Fi, WiMAX), SS7 signaling network, fixed line, local area network,wide area network, and others, including any combination of these

Example 1—PMS-Based Control of the First and Second Engines in theTransient Phase of the Aircraft Mission

In some embodiments, the methods and systems described above areperformed based on parameters received at the controller 210. Forinstance, the controller 210 may receive a weight-on-wheels (WOW) state,PMS positions for the first and second engines, and rotation speeds ofthe first and second shafts measured in real time. In this example, whenthe WOW state is true, the aircraft is on the ground and when the WOWstate is false, the aircraft has lifted off from ground and is in theair. In some embodiments, airspeed of the aircraft can be used todetermined whether it is airborne. For the sake of simplicity, in thisexample, NL1 and NL1ref denote the measured and reference rotationspeeds of the first shaft of the first engine, respectively. NL2 andNL2ref denote the measured and reference rotation speeds of the shaft ofthe second engine, respectively. Also, NL denotes a reference speedcorresponding to the rotation speed of the shaft of the engine whenoperated in the flight regime, i.e., the flight rotation speed. In thisexample, the range of rotation speeds of the placarded zone spans from65% NL to 75% NL. Accordingly, the low idle rotation speed cancorrespond to 50% NL, the high idle rotation speed can correspond to 80%NL and the flight rotation speed can correspond to 100% NL. However, therange values associated to the placarded zone can differ in some otherembodiments.

Table 1 summarizes the reference rotation speeds NL1ref and NL2ref as afunction of the PMS positions when WOW is true. These scenarios ensurethat the idling engine does not dwell in the placarded zone which isdefined at ranging from 65% NL to 75% NL in this embodiment.

TABLE 1 Reference rotation speeds NL1ref and NL2ref as a function of PMSpositions PMS PMS POSITION POSITION OF OF FIRST SECOND ENGINE ENGINENL2ref NL1ref IDLE IDLE (or OFF)  50% NL 50% NL (below the (or 0% NL)placarded zone) IDLE FLY 100% NL 80% NL (above the placarded zone) IDLETransition from Transition from Transition from 50% IDLE to FLY 50% NLto 100% NL to 80% NL at a NL at a given rate r1 rate r2 smaller than therate r1* NL1ref ≤ NL1 + 5% NL1ref ≤ NL2 IDLE Transition from Transitionfrom Transition from 80% FLY to IDLE 100% NL to 50% NL to 50% NL at arate NL at a given rate r5 r6 smaller than the rate r5** NL1ref ≤ NL2IDLE Transition from Transition from Transition from 80% FLY to OFF (or100* NL to 0% NL NL to 50% NL at a uncommanded given rate r5 flameout)*The rate r2 may be half the rate r1, e.g., r2 = 50% * r1. **The rate r6may be half the rate r5, e.g., r6 = 50% * r5.

As presented in Table 1 above, the WOW state, the PMS positions and thereference rotation speeds of the first and second engines are used bythe controller, e.g., the EEC, to select the appropriate fuel flowvalue(s) to the first and second combustor sections to ensure that themeasured rotation speeds NL1 and NL2 stay within an acceptable range ofthe associated reference rotation speeds NL1ref and NL2ref to avoiddwelling in the placarded zone.

In some embodiments, the PMS position is preferred to determine whetherthe first engine is coupled or decoupled instead of the unclutched flag(based on torque) since it allows more robust determination of theclutched/declutched states (or equivalently coupled/decoupled states)whereas using the declutched flag may lead to higher risk of togglingbetween the two states and subsequent instability on the flight rotationspeed determination.

In the first run-up phase, the first engine declutching during a run-upwill increase its NL1ref at a slower rate than the second engine toforce the first engine to declutch from the main load (e.g., whenNL1<NL2) and let the second engine steadily carry the helicopter rotorload. Similarly, the first engine clutching during a run-down willdecrease its rotation speed NL1ref at a slower rate than the secondengine to force the first engine to re-clutch to the main rotortransmission (e.g., when NL1=NL2) and avoid dwelling in the placardedspeed zone.

On turboshaft applications, it is desirable that the A/C rotor does notproduce excessive lift before take-off as the rotor speed accelerates toreach the flight rotation speed. For that reason, on ground, if a run-upis requested (PMS transition from IDLE to FLY) while collective leverposition (CLP) is above a certain value (such as 20% for instance), therun-up is inhibited. In that situation, if the first engine is in theidle regime, a basic control logic would tend to increase the firstengine's rotation speed NL1ref (from 50% to 80% NL for instance) toavoid dwelling into the placarded zone (defined from 65% to 75% NL forinstance). However, having the CLP above the threshold value willprevent the second engine from performing it's run-up (inhibited) butthe first engine will believe it is performing a run-up since the secondengine's PMS position is at the FLY position. This could lead to asituation where the first engine would increase its rotation speedNL1ref from 50% NL to 80% NL believing it will decouple while the secondkeeps its rotation speed NL2 at 50%. This would hence lead the firstengine to take the A/C rotor load and perform a partial run-up to 80% NLwhile CLP is above the threshold value therefore producing a portion ofthe lift the customer wanted to avoid.

In order to prevent this undesirable outcome as well as other potentialinteractions between the two engines in cases where the second enginecarrying the A/C rotor load is slow to spool up or fast to spool down, arequirement is added in column NL1 in Table 1 so that NL1ref may notexceed the second engine rotation speed NL2 within a given toleranceduring the second engine's transitions IDLE to FLY and FLY to IDLE.

It is also specified in column NL1ref of Table 1 that NL1ref may notexceed the rotation speed NL1 plus a given margin (5% for instance)during the second engine PMS transitions IDLE to FLY in order to preventsignificant NL variations and associated torque spikes in case therotation speed reference value gets too far above the current rotationspeed when an engine limit initially preventing the engine's rotationspeed from reaching its rotation speed reference value is lifted.

In a flight regime, i.e., when WOW is false, the first engine's NL1refis set to 80% NL regardless of the second engine's PMS position, toallow faster run-up to FLY (100% NL) of the first engine if the secondengine shuts-down. It is noted that the methods disclosed herein allowfor pro-active management of the low idle and/or high idle rotationspeeds for smooth transition between IDLE and FLY of the second engine,from a speed and a torque (clutching/declutching) point of view.Accordingly, predictability and repeatability of the first engine'srotation speed variation can be enhanced.

In some embodiments, the increase in rotation speed when the enginemoves from the idle regime to the flight regime is about +4% NL/second.In some embodiments, the decrease in rotation speed when the enginemoves from the flight regime to the idle regime is about −4% NL/second.The maximum difference between the measured rotation speeds NL1 and NL2and their corresponding reference rotation speeds NL1ref and NL2ref canbe about 5% NL when transitioning between 50% NL and 80% NL. In otherembodiments, this difference can be 6% NL or more. In some embodiments,the low idle rotation speed ranges between 50% NL and 65% NL. In someembodiments, the high rotation speed ranges between 75% NL and 80% NL.In some embodiments, it is envisaged that the methods and systemsdisclosed herein can include ranges for the low and high idle rotationspeeds as targets for each one of the transient phases of the typicalaircraft mission. Accordingly, some predetermined increases and/ordecreases in fuel flow for the first and second engines can be applieduntil the desired target ranges of rotation speeds are achieved for bothengines.

In some embodiments, a condition specifying that the rotation speed NL1can preferably remain below the rotation speed NL2 is optional forsmoother interactions between the two engines. An inhibited run-up ofthe second engine can be handled by the CLP threshold directly in someembodiments. Although it has been mentioned that the run-up and run-downphases of the aircraft are performed while the aircraft is on theground, the methods and systems described herein can be used also inflight, i.e., regardless of the WOW state, in some other embodiments. Itis noted that, in some embodiments, the toggling between the low andhigh idle rotation speeds can be based on a decoupled flag (torquesensors signal analysis) of the engines instead of the PMS position.However, using the PMS has been found to be advantageous in at leastsome situations.

Other techniques to prevent the idling engine from running into aplacarded speed zone can simply bump up the rotation speed when theresulting speed of the idling engine at IDLE fall within the placardedzone. Such techniques can present the inconvenience of not beingproactive when the non-idling engine PMS position changes from FLY toIDLE and would prevent from having smooth clutching upon both enginesreaching a common rotation speed. This technique may rely on a detectionmechanism and a placarded zone avoidance mechanism including, but notlimited to, detection mechanism based on the rotation speeds, vibrationpick-up, dynamic pressure fluctuation and the like. The placarded zoneavoidance mechanism can change minimum fuel flow, change minimum corespeed and change variable geometry (e.g., VGV, bleed valve) in order tohave the resulting rotation speed outside of the placarded zone. In someembodiments, a multi-engine system can have at least a few placardedspeed zones. In these embodiments, more than two idle rotation speedscan be defined between the placarded speed zones to avoid dwelling inany of the placarded zones. It is emphasized that the methods andsystems described herein can be applied to the rotation speed of anyshaft(s) or spool(s) of the gas turbine engine. For instance, in someembodiments, the rotation speed NL may be associated with a rotationspeed of the low pressure turbine spool. However, in some otherembodiments, the rotation speed can be associated to other types ofshafts such as the high pressure turbine spool, or any other outputshaft rotating within the gas turbine engine. In some embodiments, themulti-engine system can comprise more than two engines. In theseembodiments, instead of considering the PMS of the non-idling engine, asdescribed above, the engine having the PMS at the highest position canbe considered as the reference for the flight rotation speed NL. Assuch, the highest values of the other engines' rotation speed isconsidered instead of the rotation speed NL2 of the second engine.

Example 2—Alternative Using Measured Rotation Speeds

In some embodiments, the low idle rotation speed is denoted NL_lowidle.For instance, NL_lowidle can be 50% of the flight rotation speed NL,i.e., 50% NL. The high idle rotation speed is denoted NL_highidle, suchas 80% NL. The flight rotation speed NL can be denoted NL_flight or 100%NL for instance. In these embodiments, the rotation speed of the idlingengine, e.g., the first engine, is denoted NL_idle and defined as afunction of the second engine's rotation speed NL_nonidle to avoiddwelling in the placarded zone using an equation as per below:NL_idle=NL_lowidle+k*(max(0,NL_nonidle−NL_lowidle)),  (1)

where constant k=(NL_highidle−NL_lowidle)/(NL_flight−NL_lowidle). Forinstance, with the exemplary values presented above, k can be given by(80−50)/(100−50)=30/50=0.6.

In some embodiments, the non-idling engine's rotation speed NL_nonidleis 100% NL. In this case, the rotation speed NL_idle of the idlingengine is given by 50+0.6*(max(0, 100−50))=80% NL. This technique can beadvantageous for its simple implementation, it only requires thenon-idling engine's rotation speed NL_nonidle as additional input, andonly involves simple arithmetic operators for the idling engine'srotation speed to stay below or above the placarded zone during theoperation of the multi-engine aircraft. This technique can ensure thatthe idling engine's rotation speed stays below the non-idling engine'srotation speed while being compatible with remote engine run-upinhibition by higher CLP value. However, this technique may presentchallenges in independently tuning the rotation speed's rates of changewhen the idling engine's rotation speed's reference toggles between thelow idle rotation speed and the high idle rotation speed. In someembodiments, the idle rotation speed that is most suitable for theidling engine can be sensitive in case of rotor droop or overspeed andthe other engine's rotation speed. There can be fluctuation around theidling rotation speed reference. This could be fixed by an additionallogic that fix the idling engine's rotation speed reference to the highidle rotation speed once the transition from the second engine's PMS hasswitched from the IDLE position to the FLY position. In someembodiments, the idling engine's rotation speed NLidle is defined basedon a two-dimensional look-up table function of the other engine measuredspeeds. This method can be simple and advantageous as the impact of anyperturbation of the other engine measured speed (like rotor droop) forinstance can be cancelled by setting properly constant NLidle speed zoneclose to reference NL speed target (low idle and flight). To an extrema,having all of the possible scenarios listed above and associated areference NLidle to each scenario can be listed into a more complexmultidimensional table to take into account other parameters affectingthe idle rotation speed. FIGS. 11A and 11B show example relationshipsbetween the rotation speed of the idling engine as a function of themeasured rotation speed of the non-idling engine. As can be appreciated,any one of the first and second engines can act as the idling ornon-idling engine. For instance, in the example of FIG. 11A, the firstengine is clutched to the load and its rotation speed is measured. Therotation speed of the second engine depends on its PMS position: whenthe PMS position of the second engine is at the fly position, itsrotation speed is set to the flight rotation speed; when the PMSposition of the second engine is at the off position, its rotation speedis set to a null rotation speed; and when the PMS position of the secondengine is at the idle position, its rotation speed is given by the curveshown in solid line shown in FIG. 11A. As such, the rotation speed ofthe second engine is function of the rotation speed of the first engineas measured by some sensors in real time, for instance. In some otherembodiments, the second engine can be clutched to the load. In theselatter embodiments, the rotation speed of the first engine depends onits PMS position: when the PMS position of the first engine is at theflu position, its rotation speed is set to the flight rotation speed;when the PMS position of the first engine is at the off position, itsrotation speed is set to a null rotation speed; and when the PMSposition of the first engine is at the idle position, its rotation speedis given by the curve shown in solid line in FIG. 11B. As such, therotation speed of the first engine is function of the rotation speed ofthe second engine as measured in real time.

The embodiments described in this document provide non-limiting examplesof possible implementations of the present technology. Upon review ofthe present disclosure, a person of ordinary skill in the art willrecognize that changes may be made to the embodiments described hereinwithout departing from the scope of the present technology. There aredifferent ways of increasing the rotation speed of a shaft of aturboshaft engine which involve increasing the power to the shaft. Incombustible fuel gas turbine engines, for instance, the rotation speedcan be increased by increasing fuel flow to the combustor sections,thereby increasing the energy in the working fluid which works againstthe turbine of the shaft. In hybrid electric/combustible fuel gasturbine engines, the rotation speed can be increased either byincreasing the fuel flow to the combustor sections, or by delivering anincreased amount of power to the shaft via an electric engine, or both.The methods and systems described herein can be used with any type ofgas turbine engine including, but not limited to, turbofan engines,turboshaft engines, turboprop engines and the like. In embodiments wherethe engines are turboshaft engines, the multi-engine system can beprovided in the form of a PT6T TWIN-PAC® (-9 Series, -6 Series, -3Series) manufactured by Pratt & Whitney Canada Corp. The scalar valuespresented in this disclosure are meant to be examples only, as differentvalues for the range of rotation speeds of the placarded zone and otherparameters can be used in some other embodiments. Yet furthermodifications could be implemented by a person of ordinary skill in theart in view of the present disclosure, which modifications would bewithin the scope of the present technology.

The invention claimed is:
 1. A method of operating a multi-engine systemof an helicopter, the multi-engine system having a first turboshaftengine having a first shaft, a second turboshaft engine having a secondshaft, and a gearbox having a clutch system clutching at least one ofthe first shaft and the second shaft to a load of the helicopter, thefirst and second shafts having a range of rotation speeds defined as aplacarded zone, the method comprising: rotating the first and secondshafts at a flight rotation speed above the placarded zone when thefirst and second shafts are clutched to the load; decreasing a rotationspeed of the first shaft from the flight rotation speed to a first idlerotation speed above the placarded zone; unclutching the first shaftfrom the load during the decreasing the rotation speed of the firstshaft; and subsequently to the decreasing and the unclutching,simultaneously decreasing the rotation speeds of the first shaft and ofthe second shaft to a second idle rotation speed below the placardedzone, the simultaneously decreasing including clutching the first shaftto the load.
 2. The method of claim 1 wherein the decreasing therotation speed of the first shaft includes decreasing a fuel flow to afirst combustor section of the first turboshaft engine, and themaintaining the rotation speed of the first shaft includes maintainingthe fuel flow to the first combustor section steady when the rotationspeed of the first shaft has reached the first idle rotation speed. 3.The method of claim 1 wherein the decreasing the rotation speed of thefirst shaft is simultaneous to maintaining the rotation speed of thesecond shaft.
 4. The method of claim 1 wherein said decreasing therotation speed of the first shaft is performed progressively.
 5. Themethod of claim 4 wherein the decreasing the rotation speed of the firstshaft includes a first decrease rate and a second decrease subsequent tothe first decrease rate, the second decrease rate being steeper than thefirst decrease rate.
 6. The method of claim 5 wherein the seconddecrease rate is applied upon detecting that the first shaft hasunclutched from the load.
 7. The method of claim 5 wherein, during thedecreasing, a difference between the rotation speed of the first shaftand a rotation speed of the second shaft causes the unclutching thefirst shaft.
 8. The method of claim 1 further comprising performing themethod upon receiving a command to operate the first turboshaft enginein an idle regime and maintain the second turboshaft engine in a flightregime.
 9. A system for operating a multi-engine system of anhelicopter, the multi-engine system having a first turboshaft enginehaving a first shaft, a second turboshaft engine having a second shaft,and a gearbox having a clutch system clutching at least one of the firstshaft and the second shaft to a load of the helicopter, the first andsecond shafts having a range of rotation speeds defined as a placardedzone, the system comprising: a processing unit; and a non-transitorystorage medium having stored thereon program code executable by theprocessing unit for: rotating the first and second shafts at a flightrotation speed above the placarded zone when the first and second shaftsare clutched to the load; decreasing a rotation speed of the first shaftfrom the flight rotation speed to a first idle rotation speed above theplacarded zone; unclutching the first shaft from the load during thedecreasing the rotation speed of the first shaft; and subsequently tothe decreasing and the unclutching, simultaneously decreasing therotation speeds of the first shaft and of the second shaft to a secondidle rotation speed below the placarded zone, the simultaneouslydecreasing including clutching the first shaft to the load.
 10. Thesystem of claim 9 wherein the decreasing the rotation speed of the firstshaft includes decreasing a fuel flow to a first combustor section ofthe first turboshaft engine, and the maintaining the rotation speed ofthe first shaft includes maintaining the fuel flow to the firstcombustor section steady when the rotation speed of the first shaft hasreached the first idle rotation speed.
 11. The system of claim 9 whereinthe decreasing the rotation speed of the first shaft is simultaneous tomaintaining the rotation speed of the second shaft.
 12. The system ofclaim 9 wherein said decreasing the rotation speed of the first shaft isperformed progressively.
 13. The system of claim 12 wherein thedecreasing the rotation speed of the first shaft includes a firstdecrease rate and a second decrease subsequent to the first decreaserate, the second decrease rate being steeper than the first decreaserate.
 14. The system of claim 13 wherein the second decrease rate isapplied upon detecting that the first shaft has unclutched from theload.
 15. The system of claim 13 wherein, during the decreasing, adifference between the rotation speed of the first shaft and a rotationspeed of the second shaft causes the unclutching the first shaft. 16.The system of claim 9 further comprising performing the method uponreceiving a command to operate the first turboshaft engine in an idleregime and maintain the second turboshaft engine in a flight regime. 17.A method of operating a multi-engine system, the multi-engine systemhaving a first engine having a first shaft, a second engine having asecond shaft, and a gearbox having a clutch system clutching at leastone of the first shaft and the second shaft to a load, the first andsecond shafts having a range of rotation speeds defined as a placardedzone, the method comprising: rotating the first and second shafts at aflight rotation speed above the placarded zone when the first and secondshafts are clutched to the load; decreasing a rotation speed of thefirst shaft from the flight rotation speed to a first idle rotationspeed above the placarded zone; unclutching the first shaft from theload during the decreasing the rotation speed of the first shaft; andsubsequently to the decreasing and the unclutching, simultaneouslydecreasing the rotation speeds of the first shaft and of the secondshaft to a second idle rotation speed below the placarded zone, thesimultaneously decreasing including clutching the first shaft to theload.